From EP-A 100 119 it is known that propene can be converted by hydrogen peroxide to propene oxide if a titanium silicalite is used as catalyst. Methanol is the preferred solvent leading to high catalyst activity. Crude propene oxide obtained by this process usually contains more than 1% by weight methanol and more than 200 ppm acetaldehyde from side reactions. Depending on the reaction conditions the crude propene oxide may also contain methyl formate in amounts exceeding 200 ppm.
Most of the propene oxide is used as a starting material for polyether polyols which in turn are used to make polyurethane foams. Propene oxide for this application has to meet stringent purity requirements and the content of methanol, acetaldehyde and methyl formate has to be below 100 ppm for each component. Therefore propene oxide produced with a titanium silicalite catalyst has to be further purified and impurities of methanol, acetaldehyde and, if present, methyl formate have to be removed.
Methanol, acetaldehyde and methyl formate are difficult to remove from propene oxide by simple distillation. In mixtures containing more than 98 mol-% propene oxide these compounds show virtually the same relative volatility than propene oxide. Therefore distillative purification down to low levels of methanol, acetaldehyde and methyl formate requires columns with large numbers of separation stages operated at high reflux ratios. This leads to uneconomical investment and energy costs.
Numerous methods have been suggested to remove methanol, acetaldehyde and methyl formate from propene oxide. Extractive distillation is an established method to purify propene oxide and remove oxygenated impurities.
EP-A 1 009 746 discloses the purification of propene oxide containing methanol and acetaldehyde by extractive distillation with a polar extraction solvent having a hydroxy functionality. The process also removes part of the acetaldehyde contained in the crude propene oxide. However, as can be seen from the examples, the purified propene oxide still contains acetaldehyde and methanol in amounts exceeding 100 ppm. Therefore further purification steps are necessary to achieve the desired propene oxide purity.
EP-B 004 019 discloses an alternative to extractive distillation for the removal of carbonyl compounds from propene oxide by subjecting the crude propene oxide to a simple distillation and feeding a compound containing an unsubstituted NH2 group to the distillation column at a point above the feed point of the crude propene oxide. Liquid compounds with an NH2 group, such as hydrazine and hydrazine hydrate are fed as such. Solid compounds with an NH2 group are fed dissolved in an inert solvent. The document also describes that a solvent contained in the crude propene oxide can be separated from propene oxide in the same distillation step. However, the document contains no information or indication that methanol can be separated from propene oxide by the disclosed process.
All the solvents disclosed in EP-B 004 019 in column 7 lines 8 to 12 are of the type that can be easily separated by simple distillation from propene oxide because of sufficient differences in volatility. From the fact, that there is only a very small difference between the volatility of methanol and propene oxide at high propene oxide concentrations, a skilled person can conclude, that the process disclosed in EP-B 004 019 comprising a simple distillation is not capable to remove methanol down to the desired level of less than 100 ppm when operated at economically viable values for the number of separation stages and reflux ratio.
EP-B 004 019 also teaches that aqueous solutions of hydrazine have certain disadvantages when used for the removal of acetaldehyde: reaction of aldehyde is slow and reaction with acetaldehyde leads to insoluble products. In the propene oxide purification process disclosed in EP-B 004 019 slow reaction of aldehydes will lead to incomplete removal of acetaldehyde and the formation of insoluble products will lead to undesired deposits in the distillation column and the bottoms reboiler.
U.S. Pat. No. 2,622,060 discloses a process for the purification of a crude propene oxide by extractive distillation using an aqueous solution of an alkaline compound, such as sodium hydroxide, as the extraction solvent. Although efficient for the removal of methyl formate by saponification the process has serious drawbacks. When operated as a batch distillation it leads to excessive loss of propene oxide by propene oxide saponification. When operated as a continuous distillation purification is inefficient with the purified propene oxide containing more than 1000 ppm acetaldehyde and methanol each and having a purity of not more than 97%. Therefore further distillation steps are necessary to purify the propene oxide.
The known methods for the purification of a crude propene oxide all have a disadvantage in that they need more than one distillation step to purify a crude propene oxide containing more than 1% by weight methanol and more than 200 ppm acetaldehyde to the desired purity of less than 100 ppm methanol and acetaldehyde each.
Therefore it is an object of the present invention to provide a process for the purification of a crude propene oxide containing more than 1% by weight methanol and more than 200 ppm acetaldehyde to give a purified propene oxide containing less than 100 ppm methanol and less than 100 ppm acetaldehyde using only one distillation step.